1. Field of the Invention
The present invention relates to a pressure reservoir for exerting pressure on a hydraulic system, with which preferably a gas exchange valve of an internal combustion engine is actuated, having a housing and a piston prestressed in operation by a device.
2. Description of the Prior Art
A hydraulic system with a pressure reservoir of the type with which this invention is concerned is known from German Patent Disclosure DE 193 26 047 A1. A hydraulic system of this kind is used for instance for actuating the inlet and outlet valves of an internal combustion engine, if the engine does not have a camshaft. Such an engine has the advantage that the control times of the inlet and outlet valves are independent of the position of the piston of the applicable cylinder. Depending on the engine operating state, such as high rpm, and on the torque desired by the driver, valve opening and closing times can be achieved which make especially optimal engine operation possible in terms of emissions and fuel consumption.
The known hydraulic system functions with a hydraulic circuit, which is supplied from a hydraulic reservoir via a high-pressure hydraulic pump. An actuating device includes a piston that can be acted upon hydraulically in both directions of motion and that is connected to the valve shaft of a gas exchange valve, such as an inlet valve. Via 2/2-way valves, one at a time of the two chambers of the hydraulic cylinder can be subjected to higher pressure, which leads to a corresponding motion of the piston and as a result to an opening or closing event of the gas exchange valve of the engine block.
The hydraulic circuit communicates with a hydraulic pressure reservoir, which is embodied as a spring-loaded piston reservoir and serves to damp vibration in the hydraulic system. An identically embodied emergency pressure reservoir also communicates with one of the two chambers in the hydraulic cylinder; if the pressure drops in the hydraulic line, this emergency pressure reservoir still furnishes sufficient pressure and a sufficient fluid volume to enable the gas exchange valve to be moved to its closed position of repose. The two pressure reservoirs operate at different pressure levels, which are set by means of different stiffnesses of the corresponding restoring springs. From DE 198 26 047 A1, it is also known to use only a single pressure reservoir, which functions simultaneously as both a working pressure reservoir and an emergency pressure reservoir.
If only a single pressure reservoir is provided, its design must be such that at minimal operating pressure in the hydraulic system, sufficient hydraulic medium is stored to enable reliably moving the gas exchange valve into the closed position of repose in the event of an emergency. This requires a relatively soft spring and a long spring travel. In order at the same time to assure that over the entire operating pressure range, a sufficient damping action exists, this kind of pressure reservoir, equipped with a soft spring, must be very long structurally, as a function of the minimum and maximum operating pressure. Such a large pressure reservoir, however, can be accommodated only with difficulty in the available installation space in an internal combustion engine. Moreover, because of the great structural length, in the operating pressure range a relatively large volume of fluid must be stored in such a pressure reservoir, and as an idle volume, beyond the desired damping action, this adversely affects the dynamics of the hydraulic system.
It is therefore the object of the present invention to refine a pressure reservoir of the type defined at the outset such that on the one hand, a pressure damping function and on the other an emergency pressure function are available, while nevertheless the pressure reservoir is as small as possible.
The above and other objects and advantages are attained, in a pressure reservoir of the type defined at the outset, by providing that the device which prestresses the piston of the pressure reservoir has a characteristic force-travel curve, in one range of motion of the piston, that has a slope which differs from the slope in a different range of motion of the piston.
According to the invention, a prestressing device with a nonlinear characteristic is used in the pressure reservoir. It is understood then that first, when the piston is urged out of its pressureless position of repose, a softer characteristic of the prestressing device is desired; that is, a change in pressure results in a relatively long movement distance of the piston. In a range of motion of the piston that is far away from the position of repose of the piston, conversely, a stiffer characteristic of the prestressing device of the piston is desired; that is, a pressure change should cause only a comparatively slight motion of the piston.
In this way, both desired functions, namely the emergency pressure function and the vibration damping function, can be achieved in a single pressure reservoir: The emergency pressure function is available in the range of motion of the piston of the pressure reservoir in which the prestressing device has a relatively soft characteristic. Within this piston range of motion, the pressure reservoir is thus already capable, at only a slight pressure drop, of dispensing a large enough fluid volume into the hydraulic circuit for securing, for instance a gas exchange valve, in the event of a pressure loss. The vibration damping function exists in the range of motion of the piston within which the characteristic force-travel curve is comparatively steep. In this piston range of motion, even major pressure fluctuations result in only a slight piston motion. Accordingly, in this piston range of motion, it is also possible for only a slight movement distance of the prestressing device to be provided, which in turn is favorable for the sake of a short structural length of the pressure reservoir.
The pressure reservoir of the invention can accordingly be used on the one hand for storing a fluid volume for emergency operation, and on the other, it can be used in normal operation for vibration damping, and at the same time is very small in size. It can therefore be integrated easily and without problems into the available installation space. Furthermore, because of the slight fluid volume stored and the great stiffness of the prestressing device, an optimal vibration damping can be achieved in normal operation without impairing the system dynamics.
In a first refinement, the device which prestresses the piston of the pressure reservoir has at least two series-connected devices, with characteristic force-travel curves of different slope, which prestress the piston in operation. The desired properties of such a pressure reservoir can be achieved especially easily, since in it, the various functions are also performed physically separately.
It is especially preferred that the devices for prestressing the piston include at least two series-connected springs, and the stiffness of one spring differs from that of the other spring. A pressure reservoir with this kind of two-stage spring assembly can be constructed simply and very economically and furthermore is robust.
In an especially preferred feature of the pressure reservoir of the invention, the pressure reservoir has an elongated part with two end portions and one support portion, which is disposed between the end portions and has a larger outer dimension than the end portions and on which two adjacent springs are braced, the one spring being tightened in operation between one side of the support portion and the piston, and the other spring being tightened between the other side of the support portion and a housing portion. An elongated part of this kind enables the secure guidance of the piston, on the one hand, and of the corresponding springs, on the other.
It is also provided that at least two stops are provided, which prevent the springs from being tightened into a block in operation. Essentially, tightening springs into a block has two disadvantages: First, most springs, in the range of motion located just before tightening into a block occurs, exhibit a markedly nonlinear, and above all often non-replicable, characteristic curve behavior. This is unwanted in the present case as well. Furthermore, whenever the springs are tightened into a block, wear of the touching surfaces of the springs can occur, which can impair the service life of the springs. The stops according to the invention prevent this.
Especially simply, such stops can be realized in conjunction with the above-described elongated part: In this case, the length of the elongated part can be adapted such that one axial end of the elongated part forms a stop with a housing portion of the pressure reservoir, and the other axial end of the elongated part forms a stop with the piston.
Basically, all types of springs are suitable for the pressure reservoir of the invention. Examples are spiral springs, air springs and magnet springs. It is especially preferred, however, that at least one of the springs is a cup spring. The use of cup springs, because of the better ratio between the spring work and the installation space, brings about a further reduction in the structural length of the pressure reservoir. Moreover, because of the strong friction damping in a cup spring assembly, the damping action of the reservoir is enhanced.
The invention also relates to a hydraulic system for actuating a gas exchange valve of an internal combustion engine, in particular of a motor vehicle, having a fluid reservoir, a fluid pump, a fluid line, a pressure reservoir that communicates with the fluid line having a housing and a piston prestressed in operation by a device, and having an actuating device, which communicates via a valve device with the fluid line and actuates the gas exchange valve.
To reduce the overall dimensions of the hydraulic system, it is proposed that the pressure reservoir be embodied as described above.